1. Field of the Invention
The present invention relates to contamination detection means. More particularly, the invention relates to contamination detection means in a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table provided with a support surface for supporting and holding a substrate at its backside surface; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of the Related Art
An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target portion comprising one or more dies on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist) on its top surface. In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion at once; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
Lithographic apparatus may employ various types of projection radiation, such as ultra-violet light (UV), extreme UV, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a xe2x80x9clensxe2x80x9d. The apparatus may comprise components that are operated in vacuum, and are correspondingly vacuum-compatible.
When a substrate is loaded onto the support surface of the substrate table, the substrate may be sucked against the support surface applying a vacuum to a space between the support surface and the backside surface of the substrate. Using this method the substrate will take a form determined by the support surface. The support surface may be provided with a matrix arrangement of protrusions, substantially perpendicular to the support surface whereby the backside of the substrate is lying on a contact surface represented by a top part of the protrusions. The form of the substrate will then be determined by the contact surfaces that are all located in the same plane. If contamination particles are present in between the support surface and the backside surface of the substrate the form of the substrate is not determined only by the form of the support surface but also by the contamination particles. The contamination could cause an unacceptable deformation of the substrate, which may result in focus and overlay errors during imaging of the pattern in the mask onto a top surface of the substrate. The errors may result in a rejection of manufactured substrates and thus in a lower throughput of the lithographic apparatus, which leads to an increase in the cost of ownership.
It is an object of the invention to prevent focus and overlay errors caused by contamination particles present in between the support surface and the substrate backside surface. These and other objects are achieved in an apparatus as specified in the opening paragraph, characterized in that the apparatus comprises contamination detection means constructed and arranged to detect the presence of contamination on one or both of the support surface and the substrate backside surface.
By detecting the presence of contamination on the support surface one may initiate cleaning of the support surface such that focus and overlay errors caused by said contamination are anticipated. By detecting the presence of contamination on the backside surface of the substrate one may choose to reject contaminated substrates such that the contamination will not reach the support surface. In both manners fewer manufactured substrates will be rejected and the cost of ownership of the apparatus will be improved.
In a first embodiment of the invention, said contamination detection means comprises:
level sensing means constructed and arranged to detect a surface figure of a substrate positioned upon said support surface;
data storage means for storing surface figures of at least two substrates; and
processing means adapted to compare surface figures of at least two substrates stored in said data storage means so as to recognize a recurring deformation in the surface figures of said substrates at a similar location indicating the presence of contamination at that location on the support surface.
In this way level sensing means already present in the apparatus may be used for the detection of contamination present at the support surface. Such level sensing means may detect the surface figure of a substrate positioned upon said support surface and said surface figure may be used during exposure to adjust the height and tilt of the substrate such that the substrate will be positioned within the focal plane of the projection system. In case the surface figure of more than one substrate comprises a deformation at a similar location, said deformation indicates contamination of the support surface on said location because said contamination will deform every substrate positioned upon that support surface.
In a further embodiment of the invention, the contamination detection means is adapted to detect a location of contamination on said support surface and said apparatus comprises a cleaning tool adapted to move with respect to said support surface to said location and to clean said location. In this manner contamination detected on a particular location on said support surface may be cleaned in situ by the cleaning tool and a throughput penalty is avoided because opening, inspecting, cleaning and closing of the apparatus, which may cost a lot of time in which the apparatus is not operative, is prevented. In-situ cleaning of a particular contaminated location has. the further advantage that it is not necessary to clean the whole support surface, causing wear of the cleaning tool and of the support surface.
In yet a further embodiment the cleaning tool comprises a cleaning block having a cleaning surface and is further constructed and arranged to be positioned with its cleaning surface against said support surface and to be movable with respect to and in a plane parallel to the support surface when positioned against said support surface. Such a cleaning tool is very effective in removing contamination by abrasive cleaning of the support surface. The cleaning tool and the second object table may be constructed and arranged such that the support surface is moved and the cleaning block is held stationary against the contaminated location or such that the cleaning block is moved while the second object table is held stationary. It may also be advantageous to move both the second object table and the cleaning block. The cleaning block may comprise a ceramic material, for example, alumina and titanium oxide. The cleaning block may be at least partially electrically conductive, such that a static electrical charge will not accumulate upon the cleaning block and support surface. Attraction caused by such an electrical charge between the support surface and the cleaning block will then not occur. The roughness of the cleaning surface is advantageously 0.1 xcexcm or less. The roughness is defined as the mean value of the absolute distances between the actual surface figure of the cleaning surface and the average surface figure of the cleaning surface. Such a roughness will be enough for cleaning of the contamination but will not damage the support surface.
In still a further embodiment of the invention the cleaning block comprises a sponge. It may be advantageous that said sponge is constructed and arranged to be provided with a solvent, for example acetone. The sponge may be made from polyvinyl alcohol, for example. The contaminants present on the support surface may be dissolved in the solvent and absorbed by the sponge.
In another embodiment according to the invention said cleaning tool comprises a source of radiation and radiation directing means for directing said radiation upon said support surface to crack contaminants present on the support surface. For this purpose a laser or a lamp radiating ultra-violet radiation can be used. The embodiment may be provided with purge gas means for supplying a purge gas over said support surface. The purge gas, for example an inert gas, will prevent re-contamination of the support surface and will remove the contaminants from the apparatus.
In yet another embodiment the contamination detection means comprises a substrate inspection tool constructed and arranged to inspect the backside surface of the substrate for contamination and to reject a contaminated substrate from being loaded onto the support surface. A contaminated substrate may thus be prevented from entering the (imaging part of the) lithographic apparatus. Associated contamination will not be allowed to come into contact with and remain on the support surface. The substrate may than be taken out of the process to be discarded or cleaned elsewhere.
The inspection tool may comprise a radiation source generating a beam of electromagnetic radiation, means constructed and arranged to scan the radiation beam over the backside of the substrate, and a detector constructed and arranged to detect radiation which is off-specularly scattered by contamination present on said backside. Such an inspection tool is very efficient and fast in detecting any contamination particles present on the backside surface of the substrate.
The invention also relates to a device manufacturing method comprising the steps of:
providing a substrate which is at least partially covered by a layer of radiation-sensitive material to a substrate table provided with a support surface for holding and supporting the substrate at its backside surface;
providing a mask which contains a pattern; and
using a projection beam of radiation to project an image of at least a portion of the mask pattern via a projection system onto a target portion of the layer of radiation-sensitive material,
characterized in that the method further comprises the step of detecting the presence of contamination on one or both of the support surface and the substrate backside surface.
In a manufacturing process using a lithographic projection apparatus, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.